Inkjet printers, thermal inkjet printers in particular, have come into widespread use in business and homes because of their low cost, high print quality, and colour printing capability.
In operation, drops of coloured ink are emitted onto the print medium such as paper or transparency film during a printing operation, in response to commands electronically transmitted to the printhead. These drops of ink combine on the print media to form the text and images perceived by the human eye.
Inkjet printers may use a number of different ink colours. One or more printheads may be contained in a print cartridge, which may ether contain the supply of ink for each printhead or be connected to an ink supply located off-cartridge. An inkjet printer usually can accommodate multiple cartridges. The cartridges typically are mounted side by side in a movable carriage which scans the cartridges back and forth within the printer in a forward and reward direction above the medium during printing such that the cartridges move sequentially over given locations, called pixels, arranged in a row and column format on the medium which is to be printed.
Each printhead has an arrangement of nozzles through which the ink is controllably ejected onto the print medium, and thus a certain width of the medium corresponding to the layout of the nozzles on the printhead can be printed during each scan, forming a printed swath.
The printer also has a print medium advance mechanism which moves the media relative to the printheads in a direction generally perpendicular to the movement of the carriage so that, by combining scans of the print cartridges back and forth the across the medium with the advance of the media relative to the printheads, ink can be deposited on the entire printable area of the media.
Most printers do not print all the required drops of all ink colours in all pixel locations in the swath in one single scan, or “pass”, of the printheads across the medium. Rather, multiple scans are used to deposit the full amount of ink on the medium, with the medium being advanced after each pass by only a portion of the height of the printed swath. In this way, areas of the medium can be printed in more than one pass. In a printer which uses a “multipass” print mode, only a fraction of the total drops of ink needed to completely print each section of the image is laid down in each row of the printed medium by any single pass; areas left unprinted are filed in by one or more subsequent passes.
However, the known multipass print modes are not always satisfactory. It is in particular desirable to improve the printhead reliability in multipass print modes.